[MEncoder-users] compression: 1080i versus 720p

[Raimund Berger]

Andrew Berg <bahamutzero8825 at gmail.com> writes:

> On 12/20/2009 12:42 PM, RC wrote:
>> Interlaced video is measured in fields.  NTSC is 60Hz (PAL is 50Hz) or
>> 60 fields per second.
> Right, but 60 half-height fields contain the same amount of data as 30
> full height frames, do they not? Deinterlacing does not reduce the
> amount of data. 

It does, not in terms of pixel value data if you will, but in terms of
compressability. The fields represent different points in time after
all, so you can't expect to just glue two fields together and compress
as well as a full frame representing a single point in time. Not if the
material isn't progressive in the first place.

So deinterlacing of real interlaced material does reduce the amount of
data, even in the case of motion adapative deinterlacing. Because it
aims at constructing a "smooth" full frame which disguises the mentioned
time difference and at some point would have to resort to some kind of
interpolation. Thus improving compressability on the way.

Meaning that there's always a loss when deinterlacing to 30fps, where
the exact amount of course depends on the niftyness of the
algorithm. And would be pretty hard to measure anyway in most cases.